The present invention relates to masts used for vessels, particularly sailing vessels.
Of the many motions experienced by sailing vessels, rolling is one of the most serious. What is desirable in a sailing vessel is transverse stability.
Stability can be measured by the metacentric height or righting lever. A hull in undisturbed water is essentially acted on by two resultant vertical forces. The buoyancy (displacement) factor is focused at one point called the center of buoyancy. The weight of the vessel acts at the center of gravity. When both forces act along the same vertical line, the hull is at equilibrium. When the hull is heeled to one side, the center of buoyancy moves away from the center of gravity, creating a righting moment or righting lever.
Most modern sailing vessels are designed for high initial stability. That is, during the early stages of a rolling motion, the righting moment for the hull increases rapidly causing the hull to return to equilibrium. Unfortunately, hulls designed for high initial stability during a roll, also have an increased danger zone of instability. For example, after the righting moment reaches a maximum value for a degree of heel for a particular hull, stability begins to decrease until the capsize threshold is reached. Once past this critical degree of heel, the hull will continue to roll until fully inverted. The hull generally becomes stable in the inverted position until the hull is rolled out to a point were positive stability is reached and the hull rights itself. This range of negative stability is extremely dangerous and results in the loss of life and the ultimate destruction of the vessel.
Heavier boats with a lower center of gravity aid in decreasing the range of negative stability. Still, the range of negative stability is not avoided. Moreover, the trend is towards lighter, faster boats. Unfortunately, this dramatically increases the negative stability zone.
Methods of dealing with negative stability problems include filling a mast with foam or adding buoyant sleeves around the mast. These methods are undesirable. First, any addition of weight to a mast must be compensated by increasing the ballast. For example, the addition of one pound to the top of a 44-foot mast could require an additional 44 pounds of ballast to compensate for the additional one pound to the mast. Further, additions to the outside of the mast are less efficient aerodynamically, and can only partially cover the mast in order to avoid interfering with the sails, halyards, etc. Moreover, the addition of foam alters the strength and bend characteristics of the mast. This can lead to, among other things, an increased likelihood of structural failure under certain conditions. Other methods have employed post capsize techniques, such as pumps to expel water from a hull and mast.